In cellular systems, cell planning is necessary as the frequency spectrum is limited and since it is usually necessary to re-use the given frequency spectrum. In fixed wireless cellular systems the carrier to interference ratio (C/I) is an important system parameter, which infers limitations on other system design parameters such as which particular modulation scheme can be used for transmitting signals over the air.
For the above type of systems, C/I improvements can be obtained by spatial filtering, e.g. utilizing fixed multi-beam antennas in cellular systems.
EP1176839 shows a point to multipoint system having sectorised antennas splitting the cells into 90° sectors. By “slanting the hubs”, please confer FIG. 12 and 13 of EP1176839, it should be possible to minimize co-polar interference.
WO9721309 shows a cell plan using 180° sectors.
U.S. Pat. No. 5838670 shows a point to multipoint radio access microwave system of which FIG. 2A and 2c show cell plans consisting of two unique cell configurations that are periodically distributed over a geographic area. The cell pattern has a reuse distance of two. The cell configurations of FIG. 2A make use of 90° sectors whereby the same channel is used in opposite directions. The structure of the cell configuration of FIG. 2C makes use of 45° sectors and has been reproduced as FIG. 2 of the present document.
FIG. 1 shows a known cell plan according to a sector antenna point to multipoint (PMP) time division multiplex access (TDMA) system denoted MINI-LINK BAS by Ericsson, “MINI-LINK BAS Planning and Engineering Manual”, AE/LZT 111 0541 R1B, Ericsson Microwave Systems AB, 2001.
Typically, the frequency band and the number of channels are fixed according to official regulations pertaining to the site where the system is allowed to operate. So when more users demand more capacity, one way to enhance the capacity of the system is to further sectorise the cells.
This operation will require a new cell plan requiring new equipment and antennas to be mounted in the base-stations and modification of existing antennas from for instance a 90° sector pattern to a 45° sector pattern. In systems where various polarisations are used, antennas may be re-arranged so as to accomplish a change from e.g. a vertical polarisation to a horizontal polarisation. Therefore, migration properties of the above type of system should be considered.